Enable documentation build extensions

[namurphy]

Description

This pull request enables a few extensions for use in documentation builds.

Built-in extensions

• sphinx.ext.apidoc — generate API docs
• sphinx.ext.autodoc — include documentation from docstrings
• sphinx.ext.duration — show durations in documentation builds
• sphinx.ext.intersphinx — link to other projects' documentation
• sphinx.ext.mathjax — render math with MathJax
• sphinx.ext.napoleon — support numpy and google style docstrings
• sphinx.ext.viewcode — add links to highlighted source code

Third-party extensions

• notfound.extension — adds a notfound page
• sphinx_copybutton — adds a button that enables code to be copied

Motivation

Much important functionality in Sphinx is not enabled by default, so it is necessary to enable them specifically. For example, sphinx.ext.autodoc is what gets used to put documentation for individual functions and classes in the documentation for the project.